playing football. The students behind the commentators at Gameday are as integral to the show as the extras are in a movie. It's important that they're there, because it would be weird without them, but all they need to do to meet the show's needs is show up. And they seem to be in little danger of not showing up. The chance of being on TV is enough to draw a sufficiently large crowd.
Watching on TV, there's very little difference in the product with a hyper-engaged crowd and a normal one. It's just a crowd. They generally will cheer or boo reliably regardless of how well the commentators are connecting with them.